Quality Assurance: Planning is Valuable

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By Zulki Khan, NexLogic Technologies Inc.

There are two ways to perform quality assurance (QA). One is to move a PCB through design, fabrication, and assembly focusing on expediting the project, but leaving QA at the end of the process. Time is definitely saved by placing QA as an afterthought, but too much of the process is left questionable.

The second, better way is to apply QA in small, thought-out, and incremental steps throughout the product build cycle process. This makes the various major steps and processes measured and repeatable. Embedded QA at various process locations tells you how yields likely will be at the final stage. Performing first article check at multiple stages creates fewer surprises at the end of the product build. The first approach is fraught with numerous potential issues.

Embedding QA into the ProcessFirst, at the layout design stage, component data must be carefully and closely reviewed, with a figurative magnifying glass. For example, component data from a manufacturer can use different measurement units. One component's data may be based on the metric system; another on the conventional U.S. or British (Imperial) system. A QA step here prevents simple design errors at the component creation stage, which would have been costly later.Determining the correct viewing side of a component also bears close scrutiny when reading the data. Users can mistake the bottom side of a component for the top, if not verified properly.

These concerns may seem unimportant to the inexperienced PCB designer. However, ignoring simple things like these can adversely affect the PCB design at fabrication and/or assembly. This is especially true for circular or angular components and associated angles, arcs, and slots the PCB designer is creating. Without a proper incremental QA step here, the result is a component that doesn't correctly fit on the board, requiring special assembly creativity or a redesign. For instance, another circuit board with the correct dimensions may need to be created and piggybacked on the original PCB or the PCB may need to be scrapped and a new one fabricated at an added cost and time-to-market delay.

Part of QA is correctly performing a pad stack during component creation. Creating it involves defining connectivity within the different PCB power, ground, and routing layers. It includes covering such aspects as the decal, footprint, silkscreen, and others component aspects. Also, "keep out areas" must be defined at layout stage (Figure 1). At times, some larger components must be kept out of certain PCB areas.

60628-qafigtwo.jpgFigure 2. Internal power plane wit properly sliced power islands within the main plane.

QA must also be applied to how a PCB's power and ground are split to prevent too-small power and ground islands. QA checks that proper grounding is used so that current and voltage levels are acceptable for component placement. Otherwise, unwanted noise or crosstalk issues result. Figure 2 shows power plane splits and its islands.

Component placement is critical when splitting power and ground planes; ensuring correct group placement will maintain an efficient flow of circuitry. Also, power and ground requirements and effects must be kept in mind when splitting power and ground planes. Power and ground are dependent on component placement, and an experienced PCB layout engineer splits planes at right levels to properly connect components.

Height, mechanical, and vibration restrictions should also be covered as a QA step at component placement. In these instances, product certification and regulatory agencies such as UL/CE closely inspect for possible design flaws and will reject a product at the first sign of a problem, such as if the product could not withstand vibrations.

A sound, well-organized QA practice takes into consideration the maximum requirements defined by these agencies and beefs them up roughly 1015% percent above the requirement. By adding a "fudge factor," a design specified to meet certain regulatory demands, can, for example, incur an unusually higher current or voltage and still comply with those requirements.

A QA step is especially crucial for a design's I/O implemented through SMA, SMT, and/or thru-hole connectors and their placement on the board. Various potential defect issues surround the I/O: incorrectly designing in right angles, placing connectors on the wrong side of a board, etc. External communication becomes an issue, especially with high-speed designs.

60628-qafigthree.jpgFigure 3. Properly placed silk screen next to the components.

Properly defining and placing the silkscreen next to components (Figure 3) is another critical design factor requiring QA. Proper quality checks eliminate confusion during debug, test, field repairs, and checking for failures. Another QA check ensures proper use of decoupling capacitors next to important ICs, reducing crosstalk, noise, and ground bounce.

Near layout completion, it is wise to allocate a few hours to double check fabrication and assembly notes and assembly drawings. This QA step is vital for fabrication and assembly engineers and technicians to clearly follow a design's specifications and create a correct product. Skipping QA here dramatically increases the probability of a problem or set of problems at fabrication and/or assembly. Those problems can incur delays upwards of a week, in some extreme cases.

At layout design, a QA step must be applied to test coverage. This involves such areas as level of test coverage, test issues, type of probing to be performed on top side of the board versus the bottom, use of flying probe or ICT and whether or not sufficient coverage is given to all nets.

60628-qafigfoura.jpgFigure 4. Example of well-organized fabrication drawing with detailed notes.

FabricationFabrication notes (Figure 4) coming from PCB layout design are a good start for maintaining QA at this point in the process. Also, DRC checks on DfM issues should be applied. This means creating a sandwiched routing layer between two ground planes for noise reduction and suppression. Verifying impedance control calculations developed at layout is another part of this initial QA step at fabrication.

Complying with impedance control requirements is of paramount importance at fabrication. Figure 5 shows impedance control traces designed to match signal lengths. If this isn't performed properly, new and major costs are incurred. For example, a particular design is spec'd out at design for 5% impedance tolerance. However, after fabrication, the board's impedance tolerance is at 1215%. In this case, a QA step was not embedded at this crucial junction and the result was unacceptable, out-of-spec impedance tolerances and a less-than-optimal board. The one recourse available was to discard those PCBs and fabricate new ones, costing hundreds to thousands of dollars.

Another QC step defines the exact specification for a given surface finish. For instance, a certain weight of nickel or gold plating is required for PCB fabrication depending on the board application. QA eliminates confusion and avoids unnecessary, time-consuming questions and ambiguities.

While they may seem tedious, QA steps at fabrication verify associated fabrication materials and certifications. They include close scrutiny to defined copper volume, drill charts, plated and non-plated holes, properly defined dimensions for cutouts and slots, proper copper thieving, etch back factors for specific materials, and a host of similar areas where potential issues lurk.

AssemblyQA should be embedded at various assembly spots. Like fabrication, the starting point is assembly notes. Here, QA double checks every note to create a fully functional product. Other steps should be applied to assembly equipment, maintaining tolerance control for pick and place, wave solder, and reflow. Consider rail lengths and depths and types of mechanical, height, and process restrictions.

When boards are odd-shaped, assemblers must completely understand the need to create necessary fixtures and whether or not they can be used effectively at pick-and-place, wave solder, and test. Test procedures should also be defined as a QA step to determine whether ICT, flying probe, or functional test is required. If it is a prototype project, flying probe may only be required. But for production, an ICT fixture may be needed, especially if medical or mil/aero applications are involved.

Quality assurance should also be inextricably integrated with assembly automation. That equipment includes AOI, paste height inspection or SPI, X-ray, and other ancillary gear. QA steps in this realm are important because they provide a repeatable process, as well as measurable and quality results, eliminating DfM and DfT issues.

ConclusionWhen properly applied during design, fabrication, and assembly, QA pays handsome dividends and gives the EMS provider confidence when executing a job for OEM customers. In some camps, embedding countless QA steps may seem boring or redundant work. The extra time and conscientious efforts devoted to QA, however, comprehensively define a product build and ensure that all the necessary details are accounted for without leaving anything to chance.

Zulki Khan, founder and president, NexLogic Technologies, Inc., 2075 Zanker Road, San Jose, CA 95131 may be contacted at zk@nexlogic.com, www.nexlogic.com, (408) 436-8150 ext 102.



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